The search for compounds exhibiting enhanced anti-neoplastic activity has focused some attention on nitrosourea compounds such as 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) and related agents. Several N-(2-chloroethyl)-N-nitrosoureas (CNUs) have been evaluated clinically and have been shown to possess significant antineoplastic activity against brain tumors, colon cancer and lymphomas (See, DeVita, et al., Cancer Res. 1965, 25, 1876-1881; Nissen, et al., Cancer 1979, 43, 31-40). Characterization of the decomposition products of the clinically used CNUs, such as BCNU and 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea (CCNU), has resulted in the identification of several reactive products, including chloroethylating, carbamoylating and hydroxyethylating species (See, for example, Montgomery, et al., J. Med. Chem. 1967, 10, 668-674; Montgomery, et al., J. Med. Chem. 1975, 18, 568-571; Weinkam and Lin, J. Med. Chem. 1979, 22, 1193-1198; and Brundrett, R. B., J. Med. Chem. 1980, 23, 1245-1247).
The antitumor activity of the CNUs has been suggested to result from chloroethylation and subsequent crosslinking of DNA (See Kohn, K. W. in Recent Results in Cancer Research (Eds. Carter, S. K., Sakurai, Y., and Umezawa, H.), vol. 76, p. 141, Springer, Berlin (1981)). In support of this view is the observation that many chloroethylating agents with no carbamoylating activity (e.g., clomesone, as discussed by Shealy, et al., J. Med. Chem. 1984, 27, 664-670) possess excellent antineoplastic activity. In addition, replacement of the chloro group in CNUs by a hydroxyl group has resulted in a considerable decrease in antineoplastic activity (Montgomery, J. A., personal communication; cited by Gibson, et al., Cancer Res. 1986, 46, 553-557). Furthermore, there is some evidence that hydroxyethylation of DNA is a carcinogenic and/or mutagenic event (Pelfrene, et al., J. Natl. Cancer Inst. 1976, 56, 445-446; and Swenson, et al., J. Natl. Cancer Inst. 1979, 63, 1469-1473).
While hydroxyethylation seems to have no salutary effect on the antineoplastic activity of the CNUs, there appears to be some uncertainty regarding the role played by the carbamoylating species (i.e., the isocyanate). The isocyanate generated from the CNUs reacts with thiol and amine functionalities on proteins and inhibits DNA polymerase (Baril, et al., Cancer Res. 1975, 35, 1-5.), the repair of DNA strand breaks (Kann, et al., Cancer Res. 1974, 34, 398-402), and RNA synthesis and processing (Kann, et al., Cancer Res. 1974, 34, 1982-1988). In addition, BCNU has been shown to inhibit glutathione reductase, ribonucleotide reductase and thioredoxin reductase (Schallreuter, et al., Biochim. Biophys. Acta 1990, 1054, 14-20). Although it is believed by many that some of these same properties contribute to the toxic side effects of CNUs (Colvin, et al., Biochem. Pharmacol. 1976, 25, 695-699; Wheeler, et al., Cancer Res. 1974, 34, 194-200; and Panasci, et al., Cancer Res. 1977, 37, 2615-2618), it is entirely possible, as speculated by Gibson and Hickman (Gibson and Hickman, Biochem. Pharmacol. 1982, 31, 2795-2800) in their study of the effects of BCNU on the TLX tumor in mice, that intracellular release of isocyanates plays a role in modulating the biological activity of the CNUs against some specific tumor types. Caracemide, an investigational antitumor agent developed by the Dow Chemical Company (Newman and Farquhar, Invest. New Drugs 1987, 5, 267-271 and Slatter, et al., Chem. Res. Toxicol. 1993, 6, 335-340) is thought to act as a latent form of methyl isocyanate. This agent was shown to be active in a number of National Cancer Institute tumor models, including the mammary MX-1 and colon CX-1 human tumor xenografts implanted in the subrenal capsules of athymic mice (Clinical brochure "Caracemide NSC 253272", Division of Cancer Treatment, National Cancer Institute, 1983).
The hydroxyethylating species generated from the CNUs, 2-hydroxyethyldiazohydroxide, is thought to be formed from 4,5-dihydro-1,2,3-oxadiazole which, in turn, has been hypothesized to be the result of an internal cyclization reaction involving the N-nitroso group (Brundrett, R. B., J. Med. Chem. 1980, 23, 1245-1247). The N-nitroso group is also involved in the enzymatic inactivation of the CNUs. For example, BCNU can be inactivated by denitrosation by liver microsomal enzymes in an NADPH-dependent reaction, with the formation of 1,3-bis(2-chloroethyl)urea (Hill, et al., Cancer Res. 1975, 35, 296-301 and Lin and Weinkam, J. Med. Chem. 1981, 24, 761-763). The denitrosation reaction is catalyzed by NADPH:cytochrome P450 reductase in the case of CCNU (Potter and Reed, Arch. Biochem. Biophys. 1982, 216, 158-169 and Potter and Reed, J. Biol. Chem. 1983, 258, 6906-6911). BCNU has also been shown to undergo glutathione-dependent denitrosation catalyzed by rat (Smith, et al., Cancer Res. 1989, 49, 2621-2625) and human (Berhane, et al., Cancer Res. 1993, 53, 4257-4261) glutathione S-transferase mu isoenzymes.
Since tumor cell-catalyzed denitrosation could conceivably be a potential mechanism of resistance to the CNUs, our aim was to synthesize a series of compounds that (a) were capable of generating a chloroethylating or methylating species; (b) were capable of forming a carbamoylating species; (c) were devoid of hydroxyethylating activity; and (d) were free from structural features that would make them highly prone to metabolic inactivation.
We believed that 2-aminocarbonyl-1,2-bis(methylsulfonyl)-1-(substituted)hydrazines (I) might satisfy the above conditions for the following reasons:
(a) Base-catalyzed elimination of compounds I would result in the formation of a chloroethylating or methylating species and a carbamoylating agent as shown below. ##STR1## (b) At least three classes of prodrugs of species II, i.e., 1,2-bis(methylsulfonyl)-1-(2-chloroethyl)hydrazine, 1-(2-chloroethyl)-1,2,2-tris(methylsulfonyl)hydrazine (Shyam, et al., J. Med. Chem. 1990, 33, 2259-2264), and 1-acyl-1,2-bis(methylsulfonyl)-2-(2-chloroethyl)hydrazine (Shyam, et al., J. Med. Chem. 1993, 36, 3496-3502), with potent antitumor activity, have been identified. PA1 (c) The formation of a 4,5-dihydro-1,2,3-oxadiazole intermediate may be prevented by the absence of an N-nitroso moiety. This, in turn, may prevent the formation of a 2-hydroxyethylating agent. The absence of an N-nitroso group may also make the compounds less prone to metabolic inactivation. PA1 Y is --CH.sub.3 or --CH.sub.2 CH.sub.2 Cl, and PA1 R is C.sub.1 -C.sub.7 alkyl, cyclohexyl, methylcyclohexyl, --CH.sub.2 CH.dbd.CH.sub.2, --CH.sub.2 CH.sub.2 Cl, --CH.sub.2 CH.sub.2 CH.sub.2 Cl, --CH.sub.2 COOC.sub.2 H.sub.5, --CH(CH.sub.3)COOC.sub.2 H.sub.5 or --CH(CH.sub.2 C.sub.6 H.sub.5)COOC.sub.2 H.sub.5. PA1 Y is --CH.sub.3 or --CH.sub.2 CH.sub.2 Cl; and PA1 R is C.sub.1 -C.sub.7 alkyl, cyclohexyl, methylcyclohexyl, --CH.sub.2 CH.dbd.CH.sub.2, --CH.sub.2 CH.sub.2 Cl, --CH.sub.2 CH.sub.2 CH.sub.2 Cl, --CH.sub.2 COOC.sub.2 H.sub.5, --CH(CH.sub.3)COOC.sub.2 H.sub.5 or --CH(CH.sub.2 C.sub.6 H.sub.5)COOC.sub.2 H.sub.5. PA1 V. R=--CH.sub.2 CH=CH.sub.2 PA1 VI. R=--CH.sub.2 CH.sub.2 CH.sub.2 Cl PA1 VII. R=--CH.sub.2 COOC.sub.2 H.sub.5 PA1 VIII. R=--CH(CH.sub.3)COOC.sub.2 H.sub.5 PA1 IX. R=--CH(CH.sub.2 C.sub.6 H.sub.5)COOC.sub.2 H.sub.5 PA1 X. R=--C.sub.2 -C.sub.7 alkyl, cyclohexyl or methylcyclohexyl